In the production of woodpulp in accordance with the sulphate process, the wood is treated in the form of wood chips in an alkaline solution, which mainly consists of NaOH and NaHS, in a cooking process at a temperature which in the final stage lies in the range of 150-175.degree. C.
Upon separation of the cellulose fibres which have been detached in the cooking process, a solution of a recycled liquor is obtained, which is also called black liquor, and which in addition to the inorganic chemicals charged in the cooking process also contains from the wood released organic substanses, which mainly consist of lignin.
Thereafter, the black liquor is evaporated to a dry solids content which makes it possible to burn the organic contents without the need of supplying supplementary auxiliary fuel to the combustion plant.
After evaporation of the black liquor, it is transported to a liquor combustion plant, which normally is a so called recovery boiler, but can also be a liquor gasification plant.
In a recovery boiler the burning of the black liquor is mainly effected in the furnace cavity which comprises an upper oxidation zone and a lower reduction zone, the temperature being higher in the oxidation zone than in the reduction zone.
In the combustion of the organic part of the black liquor the chemical energy is taken care of by the transformation to heat energy and electrical energy, and the inorganic chemicals are recovered. Normally, the sulphur content is converted to an active form for the cooking process, i.e. to sulphide. The portion of sodium which is not bound to the sulphur exists mainly in the form of carbonate.
The inorganic chemicals leave the combustion process mainly in melted form (e.g. at 750-850.degree. C.) and are further processed for recovery of the chemicals.
In such a further processing there is i.a. formed sparingly soluble CaCO.sub.3, which is called lime sludge. Said lime sludge is washed with water prior to reburning in a rotary kiln, whereby a weakly alkaline solution (weak liquor) is obtained. The reburning of the lime sludge, i.e. the conversion of CaCO.sub.3 into CaO, is energy-consuming and puts high demands on the quality of the fuel, so that a high flashing point can be maintained in the calcination zone of the rotary kiln. Normally approximately 40-45% of the effective heat of the fuel is consumed by the calcination reaction, 15-20% of the consumed fuel being lost by heat radiation from the mantle surface of the kiln and the lime cooler.
In experiments aiming at an improvement of the utilization of a recovery boiler it has now surprisingly been found that the recovery boiler may be used for simultaneous combustion of the black liquor and the processing of lime sludge. Most surprising is however that the course of combustion is modified in the furnace cavity when black liquor is burnt in the presence of lime sludge, so that the temperature in said furnace cavity is raised even though the conversion of CaCO.sub.3 to CaO is endothermic.